TITLE: List of open problems of formal languages
QUESTION [5 upvotes]: As we know, there are some open problems of formal languages. I am wondering if there is a somehow complete list of open problem of formal languages. If there isn't such a list, can we make it one as answer?

REPLY [9 votes]: Here is a link which describes some of the open problems:

https://cs.uwaterloo.ca/~shallit/Talks/open10r.pdf